Most any facility that produces electric power by a generator driven by an internal combustion engine also produces a supply of CO.sub.2 that usually is vented to the atmosphere and discarded. The CO.sub.2 supply is contained in the exhaust gases produced by the engine, along with other components such as water, nitrogen, and oxygen. However, there is an ever increasing need for CO.sub.2 in tertiary or enhanced oil recovery operations and well stimulation, and in food processing and beverage bottling, to name but a few industrial uses. In addition, for a number of years now, public attention has been focused upon air quality through clean-up of emissions of exhaust gases. However most efforts have involved the installation of expensive technology that has increased the overall cost of the equipment and its operation, without producing any by-product considered to be of value. The increasing demand for CO.sub.2 along with increasing environmental pressure to reduce emissions has produced a long-felt need to which the present invention is directed.
Carbon dioxide in quantities sufficiently large enough for commercial exploitation generally has come from three sources. One such source is the naturally occurring underground supply of carbon dioxide in areas such as Colorado, Wyoming, Mississippi, and other areas. A second source is that resulting from by-products of the operation of a primary process, such as the manufacture of ammonia or a hydrogen reformer. A third source is found in the exhaust gases from burning of various hydrocarbon fuels. One of the largest problems that is faced by carbon dioxide users is the problem of transportation from the place of production to the point of use. Of the three sources discussed above, applicant believes that only the last mentioned source provides the flexibility of supplying carbon dioxide close to the demand, thereby eliminating the transportion problem. The present invention also embodies economic advantages under current Federal laws such as the Public Utilities Regulatory Policies Act of 1978 (PURPA), because the facility can be a qualified cogeneration system combined with a means to recover useful amounts of carbon dioxide from the exhaust gas of the cogeneration prime mover. The invention operates to cause a sizable reduction in exhaust pollutants, and thus represents a major advance in the efficient use of a valuable natural resource, hydrocarbon fuel. The present invention also contemplates the use of an efficient, oxygen tolerant solvent in the recovery of carbon dioxide from the exhaust gas of a prime mover.
By far the largest contributor to CO.sub.2 demand is enhanced oil recovery (EOR) processes and systems. The EOR market, while heavily influenced by the price of oil, is not entirely dependent upon it. As an oil field is depleted, there is an optimum point at which EOR techniques should be applied, and hence there is an incentive to use EOR even when oil prices are low. It has been projected that carbon dioxide from natural sources can fill only a small percentage (about 13%) of projected demand. The remaining demand must be met by other sources, such as the recovery of CO.sub.2 from exhaust gas streams according to this invention. There are literally hundreds of viable EOR projects in areas of the U.S. and Canada where natural sources of CO.sub.2 are not available. The present invention offers an effective and economical method of providing electricity to oil fields, as well as electricity, steam and exhaust gas to the separation plant. With this arrangement, CO.sub.2 can be produced at very low cost, particularly where the prime mover is fueled by natural gas which exists at the same location.
The largest non-oil field use for CO.sub.2 is in connection with refrigeration of quick frozen meats and vegetables, and the refrigeration of foods for storage and transportation. Both liquid CO.sub.2 and solid CO.sub.2 (dry ice) are used for these purposes. Another large non-oil field use of CO.sub.2 is for beverage carbonation, which is a growing market. The present invention has application to these markets as well.
There are a number of alternatives presently available for recovering CO.sub.2 from dilute, low pressure, oxygen-bearing streams. The more common techniques require compression of the gas to at least 100 psi and then removing all the oxygen. In order to accommodate the dilute CO.sub.2 concentration in the exhaust gas stream, high solvent concentrations have been required. High circulation rates are costly, and high solvent concentrations require corrosion protection and increased material costs for plant construction. Thus the use of certain solvents has not been economically competitive, and previous process designs have been capable of absorbing up to only about 75% of the available CO.sub.2 in the exhaust gas. The present invention contemplates the use of a solvent that enables recovery of in excess of 90% of the available CO.sub.2 without removing the oxygen and without compressing the exhaust gas above the pressure required to pass it through the absorption stage.
Applicant has discovered that a solvent sold under the trade designation "GAS/SPEC FT" by the Dow Chemical Company, Midland, Mich., has characteristics that permit it to carry what would normally be prohibitive levels of CO.sub.2 without causing high metal corrosion rates or solvent degradation. In the past, vapor spaces have been left unprotected when inhibitors are used. However with use of the solvent identified above, the vapor offers the same level of corrosion protection as that in the liquid circuit. Corrosion is so minimized that carbon steel can be used in most parts of the plant. No special handling is necessary to remove or to limit the amount of oxygen that enters the plant, which eliminates or reduces the capital requirements of installation of oxygen removal equipment. Thus the use of the enhanced solvent permits a sharp reduction in capital investment due to lower circulation rates (allows smaller plant), and corrosion protection that permits carbon steel to be used instead of more expensive stainless steels. With these advantages, a larger regenerator can be built for the same cost and efficiency as would otherwise be possible. Numerous other advantages are realized in the way of operating cost savings, less energy requirements due to reduced heat energy needs, solvent cost reduction, and lower maintenance costs. The present invention is particularly applicable to an engine-generator set that has an exhaust gas stream containing CO.sub.2, and which produces electrical power for running pumps, blowers, and compressors, and which produces heat energy that is used in the CO.sub.2 separation phase.
Those familiar with the art will recognize that in 1978, Congress passed the Public Utility Regulatory Policies Act (PURPA). This law requires that a public utility purchase electrical power from a "qualified facility" at that facility's "avoided cost". To receive qualified facility status, a specified portion of the available thermal energy resulting from the generation of electric power must be used in another process rather than wasted. Avoided cost is defined as the price the utility must pay on the market for purchased power to satisfy any demand which exceeds its own generating capacity. If enough of the waste heat is converted to usable energy, the unit will qualify as a "cogeneration" facility under PURPA.
There are a number of benefits that are associated with being classified as a cogeneration facility. In addition to tax credits and accelerated depreciation benefits, a public utility must purchase the power generated in a cogeneration plant, and must allow connection to their power transmission network. Other benefits have to do with the fact that a cogeneration facility is exempt from most State and Federal laws regulating public utilities.
From the foregoing it will be recognized that the present invention, which may be considered to be a trigeneration process, can generate power, extract carbon dioxide, and use the heat energy produced to activate the recovery process to recover valuable CO.sub.2 which would otherwise be wasted. Prior systems have been deficient because they were energy intensive, required expensive corrosion resistant materials, were uneconomical, polluted the atmosphere, and required significant capital investment. Applicant's recognition of the need to provide a source of CO.sub.2 at the location of need and use lead to the making of the present invention.
It is therefore an object of the present invention to prove a new and improved process and plant for CO.sub.2 generation where the above-described disadvantages in the prior art are diminished or eliminated altogether.
Another object of the present invention is to provide a new and improved CO.sub.2 production plant at the user's location.
Another object of the present invention is to provide a new and improved CO.sub.2 production process and plant including a prime mover that employs hydrocarbon fuel to drive an electric generator.
Another object of the present invention is to provide a new and improved CO.sub.2 recovery process that employs an absorbing solvent for extracting CO.sub.2 from an exhaust gas stream, such solvent having high CO.sub.2 carry capacity and low corrosiveness.
Another object of the present invention is to provide a new and improved CO.sub.2 production plant with modular installation and which is readily transportable.
Yet another object of the present invention is to provide a plant and process of the type described wherein solvent waste products can be disposed of by non-polluting incineration.